The present invention relates to a repeating system in an optical transmission system.
Optical fiber communication systems now in use utilize, as a repeating method, an optical-to-electric-to-optical conversion which involves converting an optical signal into an electric signal, amplifying and waveform-shaping the electric signal, and driving again a semiconductor laser with the amplified electric signal.
On the other hand, there has been proposed, as a new repeating method, an optical amplification method which directly amplifies an optical signal. The repeating system utilizing the optical amplification is advantageous over the conventional optical-to-electric-to-optical conversion repeating system in that repeating circuits used are simplified and that a wavelength multiplex signal and a frequency multiplex signal at a given transmission rate can be amplified collectively by one optical amplifier. Furthermore, since an amplifier gain is fixed regardless of the direction of incidence of light, optical signals in two ways can be amplified together by one optical amplifier.
The optical amplification is performed chiefly by a method using a semiconductor laser and an optical fiber Raman amplification method utilizing stimulated Raman scattering which occurs in an optical fiber. Up to now many results of studies have been reported on amplification characteristics and noise characteristics of these optical amplification methods.
Concerning the direct optical amplification system it is now predicted theoretically that a maximum amplifier gain of 30 to 40 dB is obtainable and that a signal can be transmitted, in the form of light, over 10,000 km without being affected by a background light noise caused by spontaneous scattered light, the practicality of this system as a future optical repeating system is becoming higher (IEEE. J. lightwave Tech., Vol. LT-4, pp. 1328-1333 and IEEE. J. Quantum Electron. Vol. QE-17, pp. 919-935). When this optical repeating system is put into practical use, an optical cable having a repeater will be considered as a mere optical transmission line just like a nonrepeating optical cable, and the combination of this system and an optical branching technique will make possible a multi-point communication in which a frequency (or wavelength) is allocated to each section between respective points. To permit communication between any points, it is necessary, in this instance, that signals transmitted to a branching point from respective points be turned back thereto.